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My wife picked a bike (with trainer wheels) for my 3yo son. It only has a front brake. Having gone over the handlebars once when I was just using the front brake, I'm a bit worried about this. Should I be?
(Now that the bike has arrived, it has a rear coaster brake.)
For one, your kid's bike likely has a coaster brake. Check that. If it doesn't, either add a second brake or buy a bike with a coaster brake.
Apart from the danger of going over the bars, there is also this: Every vehicle must have two independent brake systems, otherwise it's simply not fit for the road. This is independent of the type of vehicle, and bikes are no exception.
This is both a legal thing (at least in my country, and very likely in any other sane jurisdiction), and a safety issue: Brakes can, and do fail. You don't want your child sitting on its bike going downhill when the cable actuating the brake fails, and not have a backup available. You must have a backup to avoid the worst. I have completely lost track of how many braking cables I've managed to tear, and I've been very thankful for my coaster brake each time.
So, as I said, either your child's bike has a coaster brake, or you need to do something about it.
Daniel R Hicks tells me that some US states allow coaster-brake-only bikes. So, I guess, I have to exclude the US legal system from the list of sane jurisdictions. Maybe they see it as a personal freedom to be allowed to endanger yourself?
Anyway, while your state may allow you to use a coaster-brake-only bike, the safety issue remains: Chains can and do jump off the sprocket, or even break. Especially when they are older and lengthened from wear. If that happens going downhill on a coaster-brake-only bike, you are screwed. If you care about safety, one-brake bikes are just a no-no.
Small children usually do not have a grip strong enough to brake effectively with brake levers on the handle bars.
Neither do they have enough coordination to modulate brakes. This means if they were able to brake the front wheel effectively they would indeed be at risk to go over the bars.
Typically, children's bikes have coaster brakes. These allow to stop by back pedaling. Which is an elegant solution: If the child is capable of accelerating by pedaling, the child is also capable of braking by performing the inverse motion.
Everyone has to fall off their bike sooner or later. I get my kids to ride around on grass secure in the knowledge they will crash or fall and learn an important lesson before they're old and big enough to hit the road.
So while I pad them up and have helmets, I don't go overboard on worrying about their safety.
Should you worry?
Not unduly, if the child is riding on grass and reasonably dressed then they should be fine.
This is a judgement call depending on the kid, the terrain, and the bike.
My kids have balance bikes with no brakes at all. We don't ride them in hills or traffic. These bikes are a good way to learn how to steer and balance in a controlled environment.
My kids later graduated to fixed gear bikes with a front brake only. I consider the fixed gear to be "half a brake", but I would be a little nervous to have them ride a freewheel bike with only one brake. But again it depends on the terrain.
BMX racing bikes have only rear brakes. But they have no need to stop on hills, and there's no traffic or pedestrians on BMX tracks. Of course track racing bikes usually have no brakes at all besides the fixed gear.
Personally I think coaster brakes are terrible. I know there's a theory that they are better for kids but they really only exist for cost reasons. They aren't easy to use, even for kids. With the proper sized levers and grips, kids as young as 4 can use hand brakes. An under-appreciated problem is grips that are too thick. Better kids bikes such as Spawn have handlebars with 3/4" tubing instead of 7/8". I build my custom kids bikes with 1/2" tubing for any kids under 8. It is much easier for them to grip and reach brakes. When I have to use standard size tubing, I usually remove the grips and use a single layer of bar tape to reduce the diameter.
Overall I think that having two hand brakes is the best, but again it depends on the kid, the bike, and how controlled the environment is. Hand brakes are no use at all if the kid can't use them effectively when needed. Same with coaster brakes.
